macro_rules! hashmap {
    ($($key:expr => $value:expr),* $(,)?) => {
        {
            let mut m = std::collections::HashMap::new();
            $(m.insert($key, $value);)*
            m
        }
    };
}
fn main() {
    println!("Hello, world!");
    let mut m = hashmap!("a" => 1, "b" => 2, "c" => 3,);
    m.insert("d", 4);
    dbg!(m);
}
